[AMDGPU] PromoteAlloca: Handle load/store subvectors using non-constant indexes (#71505)

I assumed indexes were always ConstantInts, but that's not always the
case. They can be other things as well. We can easily handle that by
just emitting an add and let InstSimplify do the constant folding for
cases where it's really a ConstantInt.

Solves SWDEV-429935